Mass spectrometers are powerful tools for solving important analytical and biological problems. For example, mass spectrometers can be used to determine the molecular weight of an ion by measurement of its mass-to-charge (m/z) ratio, while its structure may be elucidated by dissociation methods and subsequent analysis of fragmentation patterns.
The most common useful ion sources for large molecules are atmospheric pressure chemical ionization (APCI), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) sources. In contrast to other types of ion sources, such as electron ionization or inductively-coupled plasma sources, the ionization processes used in MALDI and ESI sources may be characterized as gentle, in that molecules become charged without inducing fragmentation, thereby preserving the identity of the sample molecules. Such gentle ionization can be efficiently achieved with MALDI and ESI even for relatively large biomolecules such as proteins, peptides, DNA, RNA, and the like. This capability is in large part responsible for the important role that MALDI and ESI, coupled to mass spectrometers, have come to assume in the advancement of research and development in biotechnology fields.
In general, MALDI generates primarily singly charged ions (z=1), while ESI efficiently produces primarily multiple-charged ions (z>1) (Fenn, et al, Science 246, 64 (1989)). These different charge-state distributions lead to different advantages and disadvantages of the two ionization methods. For example, the analysis of mixtures of components is often more straightforward with MALDI due to the presence of only single-charge states, versus the more complicated multiple-charge-state distributions produced by ESI. On the other hand, specific structural information can be very difficult to obtain with MALDI for relatively large molecules (e.g., with mass >20,000 Da), because fragmentation methods commonly used to elucidate structure tend to be relatively inefficient for ions with large m/z values. Detailed information on the structure of a molecule is often at least as analytically useful, if not more so, than knowledge of the mass of the molecule.
However, even a very large molecule may be analyzed in conventional mass spectrometers if the molecule can be ionized with multiple charges. For example, if a protein of molecular weight 30,000 Da acquires 10 charges, its m/z value is reduced to 3,000, which is readily measurable with essentially all commonly used mass spectrometers. The multiple-charge ionization of large molecules is one prominent capability of the ESI process, which has resulted in rapid growth of the popularity of ESI sources for the creation of multiple-charge ions of a variety of biomolecules, including small organic molecules, peptides, proteins, and other molecular complexes such as DNA derivatives. Mass spectrometer types that have been configured with ESI sources include Fourier transform ion-cyclotron resonance (FTICR), magnetic-sector, 2-dimensional and 3-dimensional quadrupole ion-traps, quadrupole mass filters, and hybrid instruments consisting of various combinations of these types, as well as others.
An important application of ESI combined with mass spectrometry is the structural identification of peptides, proteins, and other biomolecules with amino-acid residues. Structural analysis is often performed with a so-called tandem mass spectrometer using a technique referred to as MS/MS analysis. Essentially, a precursor ion of interest is m/z-selected in a first stage of a tandem mass spectrometer, and the selected ion is then fragmented in a second stage to produce product ions. These product ions are then m/z-analyzed in a third stage, resulting in a product-ion mass spectrum that represents a fragmentation pattern of the selected precursor ion. Such tandem instruments may be configured so that the separate stages are either sequential in space, such as multiple quadrupole mass filters arranged co-axially in series, or sequential in time, as with a single three-dimensional ion trap.
Deductions about the molecular structure of the precursor ion may then be made from an analysis of the fragmentation pattern observed in the product-ion spectrum. For example, the sequence structure of a protein may be (at least partly) determined from the measured m/z values of the various detected fragment ions, by deducing the sequence of amino acid residues that would have had to exist in the protein precursor ion to produce the observed fragment ions. The ideal situation in this case would be the cleavage of the amine backbone bonds on either side of each amino acid residue in a protein or peptide chain.
The success of this approach depends fundamentally on the extent to which dissociation occurs at such strategically advantageous locations in the structure of the precursor ion. Whether dissociation occurs by cleavage of any particular chemical bond in a precursor ion depends on many factors, including: the nature of the chemical bond; the amount of energy absorbed by the precursor ion; the modes available in the precursor ion to dissipate energy; and the mechanism by which energy is deposited. The various mechanisms by which energy may be deposited in an ion have given rise to a variety of fragmentation methods, such as collisionally activated dissociation (CAD), in which energy is deposited in a precursor ion as a result of collisions with a target gas; and, infrared multi-photon dissociation (IRMPD) which involves absorption of infrared photons by the precursor ions.
While distinctly different in approach, both CAD and IRMPD depend ultimately on the excitation of vibrational and rotational states within the precursor ion to cleave chemical bonds, and so the fragmentation patterns resulting from either method naturally tend to be dominated by excitation of the lowest-energy vibrational and/or rotational states. Consequently, cleavage at some bond sites of a particular precursor ion is typically preferred over others within any particular ion. Given that only a limited amount of energy is available for ‘activation’ of an ion, and that some energy may be dissipated by exciting vibrational or rotational modes without bond cleavage, a limitation of CAD and IRMPD is that the probability for dissociation of a precursor ion by cleavage at many of its bond sites may be insignificant relative to that of other, more energetically-favored, sites. For example, for peptides, cleavage readily occurs at the N-terminal side of a proline residue or the C-terminal side of an aspartic acid, while cleavage seldom occurs at di-sulfide bonds. The net result is that the structural information provided by fragment ion spectra is often insufficient to deduce a complete residue sequence.
For small peptide precursor ions, i.e., those consisting of typically less than 10–15 amino acid residues, the dissipation of energy within an ion without bond cleavage can be relatively inefficient due to the limited number of bonds. In this case, bond cleavage may occur with sufficient probability for most, if not all, of the strategically important cleavage sites, resulting in a relatively comprehensive sequence analysis. In general, though, proteins, peptides, peptide nucleic acids (PNAs), and other biomolecules can be substantially larger than such small peptides, and, in fact, can frequently contain hundreds of amino acid residues. Owing to the much greater ability of such large ions to absorb and dissipate vibrational and rotational energy, significant cleavage with the CAD or IRMPD methods often occur only for the most energetically favored cleavage sites, resulting in relatively sparse fragmentation spectra. Consequently, the CAD or IRMPD approaches alone frequently do not provide sufficient sequence information for a complete structural analysis to be performed on many molecules.
An alternative approach to CAD or IRMPD was reported recently by Zubarev et al., in J. Am. Chem. Soc. 120, 3265 (1998), where they teach that multiple-charged ions dissociate differently upon capture of low-energy electrons than they do with CAD. In this process, called electron-capture dissociation (ECD), low-energy electrons combine with low-energy, multiple-protonated molecules in the gas phase. Unlike CAD and IRMPD, the energy for fragmentation is derived from electronic state interactions rather than by vibrational and/or rotational state excitations. Subsequent to the capture of a low-energy electron, a multiple-charged ion is believed to undergo a structural rearrangement, leading to structural instability and, ultimately, fragmentation. These processes are proposed to be sufficiently fast that competing processes, such as energy redistribution, are less likely to occur than with CAD or IRMPD, resulting in bond cleavage that is less dependent on bond strength than with CAD or IRMPD. Consequently, the fragmentation patterns generated by ECD exhibit a larger variety of different cleavage patterns than those generated by CAD or IRMPD.
The advantages of ECD, either alone, or in combination with CAD, have been amply demonstrated. For example, ECD has been found to cleave peptide backbone amine bonds, (Cα-N bonds), which cleave infrequently with CAD, and results in much greater peptide sequence coverage than with CAD. Additionally di-sulfide bonds of larger proteins readily and selectively fragment, unlike CAD. Consequently, for example, McLafferty et al., in Science 284, 1289 (1999), report that, for the 76-residue ubiquitin (8.6 kDa), data from one CAD and two ECD spectra provided complete sequence information. Olsen et al., in Rapid Commun. Mass Spectrom., 15, 969 (2001), report that the combination of CAD and ECD yields similarly powerful complementary data for sequencing peptide nucleic acids (PNAs). Horn et al., in Anal. Chem. 72, 4778 (2000) also teach that the combination of CAD and ECD, whereby ions are subjected to ECD while colliding with background gas, increases the efficiency of cleavage at least 3-fold for a smaller protein (17 kDa) and extends the usefulness of ECD to much larger proteins (>40 kDa). Therefore, it is evident from these and other reports that ECD often yields nearly complete sequence mapping of small proteins (<20 kDa) and, at the least, has been demonstrated to be a powerful complement to conventional CAD methods, even for larger ions.
Thus far, however, the success of the ECD technique has only been reported in conjunction with FTICR mass spectrometers. In an ICR cell, precursor ions are stored under the influence of magnetic and electric fields; the ions oscillate at cyclotron frequencies corresponding to their m/z values, and the Fourier transform of the repetitive signal that such m/z-dependent oscillations produce results in the measured m/z spectrum. Although the incorporation of ECD fragmentation into FTICR instruments has been relatively successful, it has not been without challenges. The first requirement for reasonable fragmentation efficiency by ECD is the production of a large flux of low-energy electrons in the energy range of <0.2 to about 5 eV. The significance of this requirement was demonstrated recently by Hakansson et al., in Anal. Chem., 73, 3605 (2001), who reported two to three orders of magnitude increase in sensitivity by optimizing the design and operation of their electron source, and subsequently by Tysbin et al., in Rapid Commun. Mass Spectrom. 15, 1849 (2001) who demonstrated the potential for rapid analysis enabled by the use of relative large indirectly heated dispenser type cathodes in the electron source.
Apart from the production of a healthy flux of low-energy electrons, a second critical requirement is to be able to transport low-energy electrons into the mass spectrometer with good efficiency. A third critical requirement is to retain low-energy electrons in the volume occupied by precursor ions long enough to allow a significant number of interactions to take place between the precursor ions and the low-energy electrons. The successful incorporation of the ECD technique in FTICR instruments is directly related to the relative ease with which low-energy electrons can be readily transported and retained, along with precursor ions, due to the stability of the electrons' motion in the strong magnetic fields of the ICR cell.
FTICR instruments, however, are currently relatively expensive, and require specialized skill to operate and maintain. Therefore, it would be of substantial benefit to incorporate the ECD fragmentation technique into more economical, commonly-used types of mass spectrometers, such as triple quadrupole mass spectrometers, quadrupole-time-of-flight mass spectrometers, two-dimensional quadrupole ion traps, and other similar multipole ion guide-based mass spectrometers. Unfortunately, in contrast to ICR cells of FTICR instruments, multipole ion guide-based mass spectrometers typically utilize only DC and AC (RF) electric fields, that is, without magnetic fields. (For this reason, such multipole ion guides are sometimes referred herein as ‘RF multipole ion guides’, which is to be understood to encompass ion guides that employ both DC and RF voltages, as well as RF-only voltages). Generally, the stability of motion of a charged particle in such electric fields extends only over a limited range of particle m/z values. However, the m/z value of an electron is typically a factor of at least five orders of magnitude less than ions with even the lowest m/z value of interest. Therefore, low-energy electrons and precursor ions are hardly likely to be stable simultaneously within the fields of an RF multipole ion guide, in contrast to the situation in an FTICR instrument.
In addition, electrospray ionization readily produces negative ions as well as protonated positive molecules, and most mass spectrometers have the capacity to routinely analyze and detect both positive and negative ions. The ECD method of fragmentation is not useful for negative ions, since the Coulomb repulsion of same-polarity charge would preclude the close-range interaction of electrons and negative ions. Nevertheless, a fragmentation method similar to ECD would prove to be just as useful for structure analysis of negative ions as ECD appears to be for positive ions. In fact, it is expected that the capture of positrons (electron anti-particles) by negative ions follows a mechanism similar to electron capture in reaction with positive ions. In analogy to ECD, the fragmentation of ions due to capture of positrons may be referred to as ‘positron capture dissociation’, or PCD. Positrons are stable but relatively short-lived due to their strong reaction with matter. However, McLuckey et al., in Rapid Commun. Mass Spectrom. 10, 269 (1996), has reported that positron capture by organic molecules can occur, and, at positron energy less than about 3 eV, extensive fragmentation of organic molecules was observed. They also noted that the fragmentation efficiency increased as the positron energy decreased, similar to trends observed with ECD fragmentation of positive ions, which seems to suggest that similar mechanisms leading to fragmentation are involved. The apparatus incorporated a Penning trap where close interaction between positrons and organic molecules was achieved in the presence of a 1 T magnetic field over the length of the trap. As with ECD, the incorporation of PCD into RF multipole ion guide-based mass spectrometers would be of substantial benefit for ion structure determination by MS/MS analysis, in particular, of negative ions.
Despite the clear desirability of performing ECD and PCD within RF multipole ion guide-based mass spectrometers, the means by which this may be accomplished has not previously been available.